Variable intensity laser: – The intensity of laser is controlled by
voltage input to the source that emits laser. Laser source emit
light at any of 16 different levels of intensities. The first level
of intensity is LEVEL 1, this state occurs when laser is off; the
sixteenth level of intensity occurs when laser is ON and emits light
with maximum intensity. 14 intermediate levels of intensities are
between Level 1 and Level 16. These 16 levels are evenly spaced (the
difference between intensities of two consecutive intensity levels
is equal). A hexadecimal number is assigned to each intensity level.
Table below shows 16 intensity levels of a laser and corresponding
hexadecimal number.

Intensity Level

Hexadecimal number

Binary Value

Level 1

0

0000

Level 2

1

0001

Level 3

2

0010

Level 4

3

0011

Level 5

4

0100

Level 6

5

0101

Level 7

6

0110

Level 8

7

0111

Level 9

8

1000

Level 10

9

1001

Level 11

A

1010

Level 12

B

1011

Level 13

C

1100

Level 14

D

1101

Level 15

E

1110

Level 16

F

1111

Intensity of laser is controlled by
voltage input to the source of laser. When the source emits laser at
specific intensity level, the corresponding hexadecimal number
assigned to that intensity level is transmitted. Four bit binary
data is transmitted by one laser since each hexadecimal number is
equivalent to 4 bit binary number.
Data transfer over optical signal through variable intensity
monochromatic lasers
A variable intensity
monochromatic laser transmits a hexadecimal number at every clock
pulse.

The optical transmitter consists of four variable
intensity monochromatic lasers. Each laser is of specific wavelength
and no two lasers are of same wavelength. The intensity of each
laser is controlled by voltage input to the source of that laser.
Four lasers with 16 intensity levels each make
164
= 65536 combinations of different intensities and wavelengths.

These combinations are sufficient to transmit 16-bit
binary number at one instance. (Since combinations of 16 bit binary
number (1s and 0s) = 216
=65536).

Construction and working of transmitter

The transmitter consists of four
lasers. Each laser is of specific wavelength. The wavelengths (colours)
are RED, YELLOW, GREEN and BLUE. All four lasers are variable
intensity lasers. The intensities of these lasers are controlled by
voltage input to laser sources. Each source has an independent
voltage input. The voltage input is provided by an input voltage
controller circuit. The circuit generates separate voltage input for
each laser depending upon hexadecimal number to be transmitted.

Sixteen bit binary data is converted
into equivalent 4-digit hexadecimal number. Most significant digit
of hexadecimal number controls input voltage of RED laser source.
The second most significant digit of hexadecimal number controls
input voltage of YELLOW laser source. The third most significant
digit of hexadecimal number controls input voltage of GREEN laser
source. And the least significant digit of hexadecimal number
controls input voltage of BLUE laser source.

The intensities of these laser change
at every clock pulse in accordance to hexadecimal number to be
transmitted. All lasers are transmitted through single optical
fiber.

Construction and
working of optical Receiver

The receiver consists of a prism. The
optical signal (light) received is made incident on the prism. The
prism divides the light in to different components depending upon
the wavelengths. Each colour component is made incident on separate
light sensor. The sensor generates electrical signal in proportion
to the intensity of light incident on it. The sensor that senses
intensity of RED light component generates electrical signal whose
voltage is proportional to intensity of RED laser. Similarly for
sensors those sense intensities of YELLOW, GREEN and BLUE components
generate electrical signals proportional to intensities of
respective components incident on the sensors. Electrical signal
generated by RED sensor represents most significant digit of
four-digit hexadecimal number received in optical form. Similarly
the electrical signals generated by YELLOW, GREEN and BLUE sensors
represent second most, third most and least significant digits of
the four digit hexadecimal number respectively. The outputs of all
sensors are connected to a circuit that converts electrical signals
into equivalent hexadecimal number. The four hexadecimal numbers
together represent a 4-digit hexadecimal number received in optical
form. The hexadecimal number is converted in to equivalent 16-bit
binary number by hexadecimal to binary converter circuit. The 16-bit
binary number obtained after conversion is 16-bit binary number
transmitted by the transmitter. This 16-bit binary number is
transmitted in one clock pulse time in optical form through single
optical fiber.

Four lasers transmit a 4-digit
hexadecimal number. The equivalent binary number transmitted at one
clock pulse is 16-bit binary number. (16 bit binary number is equal
to 2 bytes)